1. Field of the Invention
The present invention relates to a process for the production of aluminum hydroxide containing material, in particular for use in the preparation of catalyst carriers.
2. Description of the Related Art
The state of the art methods for preparing alumina include use of raw materials produced by precipitation as exemplified by mixing bases such as alkaline aluminate and alkaline hydroxide bases with an acid such as HNO3, H2SO4 and H3PO4, in addition to Al(NO3) and Al2(SO4)3. Useful carriers for refinery processing catalysts are produced from alumina and intermediate products like pseudo boehmite owing to their large surface area and high porosity.
The known methods for preparing alumina catalyst carriers arc costly to purchase the acid and basic raw materials being employed in the preparation process, and also expenses to perform pH adjustments, washing steps and wastewater treatment.
The general object of this invention is, thus, to provide an improved process for the production of alumina such as aluminum oxide and pseudo boehmite for use in the preparation of catalyst carriers, wherein the production of alumina is carried out by simplified and inexpensive steps. Compared to the known methods, the advantages of the invention include high product yield through reduced loss of material during the processing.
The process of this invention comprises the following steps:
a) dissolution of gibbsite (Al2O3.3H2O) in nitric acid to provide an acid solution of Al(NO3)3 by reaction (i):
Al2O3.3H2O+6HNO3xe2x86x922Al(NO3)3+6H2O;
b) thermal decomposition of the acid solution into aluminum hydroxides and NOx gases by reaction (ii):
2Al(NO3)3+6H2Oxe2x86x92Al2O3.ZH2O+6NO2+(6-Z)H2O+3/202;
and
c) regeneration of NOx gases in one or a series of absorption towers to concentrated nitric acid according to reaction (iii):
6NO2+3H2O+3/2O2xe2x86x926HNO3.
The overall reaction, i.e. (i)+(ii)+(iii), is then:
Al2O3.3H2O+3H2Oxe2x86x92Al2O3.ZH2O+(6-Z)H2O,
wherein
Z=0.5-3. 
In a specific embodiment of the invention, NO2 being formed in the above reaction (i) is partly regenerated in absorption towers. Reaction (ii) results in formation of nitric acid that is recycled and utilized for dissolution of gibbsite, which is the sole raw material in the process.
Due to minor loss of the nitric acid to be recycled, however, it may be occasionally necessary to add small amounts of nitric acid in addition to what is being currently regenerated, to maintain or enhance dissolution of the gibbsite raw material.
Decomposition in the above step (b) may be performed by spraying the acidic solution obtained in step (a) onto the inner surface of one or more rotary kilns with a free supply of air at 300-700xc2x0 C. However, by using this method, measures may be taken in order to prevent sticking of the product to the inner surface of the rotary kiln, e.g. by means of one or more sliding chains. In order to be efficient, the chains must be relatively heavy and are to be present inside each kiln in such a way that they slide down on the kiln surface, while the kiln rotates, and thereby prevent the solid material from adhering to the surface.
Adhesion of the prepared material from the decomposed acid solution to the inner side of said kiln may also be prevented by other physical or chemical means known in the art.
Other methods for performing the decomposition set forth in (b) above include drying the acidic solution obtained from (a) in a fluid bed, and drying the solution in a steel band conveyor furnace.
The alumina product can be further improved by addition of an organic compound capable of reducing nitrates. Those components can be glucose or similar compounds. The reaction between the organic compound and the nitrate will then generate a faster decomposition of the nitrates through minor local explosions, and, thus, reduce the size of the formed particles.